The above mentioned former art has operated for more than a decade from its origin with high reliability and performance. I have found however, that the efficiencies can be further improved and thereby the life time can be elongated and the pressure and rotary velocity in machines which employ the pivot-portion of a piston shoe in a slotted bore of a piston can still further be improved. I have also found, that such improvement is necessary in order to apply such machines for ever higher pressures, powers and velocities in from time to time advancing applications with requirements to higher power and efficiency; like aircraft, vehicles, construction machinery, machine tools and like.